Blood of the Legion
It is a perk that is added to Rex after the quest Nothin' But a Hound Dog if you use Lupa's brain for the transplant. It gives Rex an additional 10 DT (as of 18.104.22.168x patch)and 100 extra HP (pre patch).
This "perk" is actually an effect that is applied to Rex. The additional DT is the only effect that has a baseID associated with it.